Paho MQTT Python Client: No exceptions thrown, just stops

I try to setup a mqtt client in python3. This is not the first time im doing this, however i came across a rather odd behaviour. When trying to call a function, which contains a bug, from one of the callback functions (on_connect or on_message), python does not throw an exception (at least it is not printed), it just stops there. I tied together a short example, that reproduces that behaviour.

Does someone has an idea?

import paho.mqtt.client as mqtt

import re
import os.path

import json
from termcolor import colored

client = mqtt.Client()

def func():
    test = 1 + "1"
    print("Should never reach that")

def on_connect(client, userdata, flags, rc):
    """Establishes connection to broker
    """
    print("Connected to broker with result code " + str(rc))
    client.subscribe("test")

def on_message(client,userdata,msg):
    print("Recieved message on " + msg.topic)
    params = {}
    if msg.topic == "test":

        print("Invoke func")
        func()

if __name__ == "__main__":
    client.on_connect = on_connect
    client.on_message = on_message

    client.connect("localhost",1883,60)

    client.loop_forever()

JavaScript
​x
 
import paho.mqtt.client as mqtt
​
import re
import os.path
​
import json
from termcolor import colored
​
client = mqtt.Client()
​
def func():
    test = 1 + "1"
    print("Should never reach that")
​
def on_connect(client, userdata, flags, rc):
    """Establishes connection to broker
    """
    print("Connected to broker with result code " + str(rc))
    client.subscribe("test")
​
def on_message(client,userdata,msg):
    print("Recieved message on " + msg.topic)
    params = {}
    if msg.topic == "test":
​
        print("Invoke func")
        func()
​
if __name__ == "__main__":
    client.on_connect = on_connect
    client.on_message = on_message
​
    client.connect("localhost",1883,60)
​
    client.loop_forever()
​

This is the output when sending a message to the topic “test”:

Connected to broker with result code 0
Recieved message on test
Invoke func

JavaScript
 
Connected to broker with result code 0
Recieved message on test
Invoke func
​

When calling func() from main, i get the correct TypeError thrown. So something catches this exception in paho. I had a look at an olderproject (python2 though) and tried to recreate the behaviour. There the exception gets thrown correctly. What do i miss?

EDIT I can catch the exception by putting the func() call in a try block. How ever, it does not stop the execution of the program when not catched. I dont get why

Answer

This will be due to the fact that the on_message function is called by the network thread and it will be wrapping that call in a try block to stop errors in on_message from stopping that thread.

If you want to an error to stop the app then you should use your own try block in on_message and behave appropriately.

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Answer